1. Field of the Invention
The present invention relates generally to a antenna array apparatus and method, and in particular, to an apparatus and method for efficient transmission beam forming of a antenna array in a mobile communication system.
2. Description of the Related Art
In order to meet an increasing demand for code division multiple access (CDMA) mobile communication systems, active researches have been conducted on a antenna array that form beams using a plurality of base station antennas. Commercialization of the antenna array is currently being developed. Since the future mobile communication must provide various high-speed multimedia services, a forward beam forming technique for enabling high-speed data transmission is needed.
Generally, in a mobile communication system, an antenna or smart antenna technology for a base station refers to a technology for transmission beam forming in a direction of a desired mobile station, using a plurality of antenna elements. However, compared to a base station system using the conventional omni-directional antenna or sector antenna, the antenna array technology has a larger subscriber capacity per cell, wider service coverage, call quality improvement, and a reduced power consumption per power amplifier. Such a base station antenna array can be applied to both transmission and reception. Herein, a description of the base station antenna array will be made with reference to a method of transmission beam forming.
The conventional sector antenna transmits both common channel, i.e., pilot channel, synch channel and paging channel, signals and traffic channel signals in a single common beam. However, the antenna array system uses a common beam for the common channel signals, it forms a narrow traffic beam for the traffic channel signals, in order to obtain the above-stated effects.
IS-95 or CDMA2000, a mobile communication specification, uses coherent modulation such as binary phase shift keying (BPSK) or quadrature phase shift keying (QPSK), both commonly called “multiple phase shift keying (MPSK),” In case of MPSK, a phase difference or phase offset may occur between a common channel signal transmitted to a mobile station through a common beam and a traffic channel signal transmitted to the mobile station through a traffic beam. The phase difference has a detrimental affect on system performance. Therefore, it is necessary to minimize the phase difference to minimize a bit error rate (BER) in order to meet the requirement for a desired call quality.
Herein, a common beam, for transmitting common channel signals, e.g., pilot, synchronization and paging channels must be commonly transmitted to all mobile stations in a sector. Thus it must be wide enough to cover the whole region in a sector. On the contrary, each traffic beam has different look direction of arrival(DOA) and Beam Width(BW) depending on the channel environment of each mobile station.
As described above, in order to form a transmission beam in a direction of a mobile station, it is necessary to form a transmission beam after estimating the DOA and BW.